IFKP is a incremental frequency keyed mode with an offset of +1. It is designed for very high coding efficiency. This version uses 33 tones (32 differences), a Varicoded Alphabet, with a rotating difference frequency. Modem statistics: Parameter ValueSamplerate 16000Bandwidth 386Center freq 1500(1)Symbol length 4096Lower case cps 3.65(1) nominal, adjustable from 500 to 3500 Hertz

Tone differences are expressed as decimal numbers 0 - 31. The alphabet design uses single-symbol and two-symbol character codes. The character set does not include much extended ASCII, and has only 116 different codes. It is a strongly 'unsquare' design (29 x 3), which restricts the number of codes, but we don't need the maximum anyway. This 'unsquare' code maximizes the single-symbol set to 29, so will fit all lower case in one symbol. Using 29 initials means using a comparison process to discover where the initial symbols are. Amateur communications does not follow standard English text for letter frequency. It has a higher incidence of letters such as Q, X and Z (in Q codes and callsigns). Lower case letters are more common than upper case. Numbers are considered equal in frequency of occurance to upper case characters. The IFKP character set has been designed for one set of 29 single-symbol characters, and three two-symbol sets of upper case letters, number and punctuation. There is some spare space, and a few extended ASCII characters have been included (± ÷ ° £ ×). There are 29 single-symbol characters, 87 two-symbol codes, and a total alphabet of 116 codes. This allows coding all upper case, lower case and punctuation. The most common symbol is the NULL, a non printing character and is assigned a single symbol entry in the table. The nomenclature used for the character symbol numbers in the tables is

[1st_symbol], ([2nd_symbol])expressed in decimal, where the [1st_symbol] always has values less than 29, and the [2nd_symbol] will have a value of 30 or 31. IFKP Varicode

It is about 27% more efficient than DominoEX. Note especially the score sending typical 'ham speak' - some 30% more efficient than DominoEX. The typing speed at 3.65 cps is about 37 WPM. Q codes and callsigns should be sent in lower case when possible. Character rate statistics for various character sets when sent using IFKP, text: abcdefghijklmnopqrstuvwxyzchars/sec: 3.65text: ABCDEFGHIJKLMNOPQRSTUVWXYZchars/sec: 1.84text: 0123456789chars/sec: 1.84text: !#$%&*()_+-=[]{}|;:'",<>/?<bs><lf>chars/sec: 1.84IFKP xmt speed

fldigi-ifkp can transmit text in one of 3 speeds, 0.5X, 1.0X and 2.0X. The 1.0X speed is the natural speed of the modem. The 0.5X and 2.0X speeds are obtained by dividing or multiplying the symbol length by 2 respectively. You can expect to experience decreased decoding errors with the 0.5X speed. Use it when s/n conditions are well below minux 10 dB on the IFKP signal strength meter. The 2.0X speed should probably not be used unless the s/n is well above plus 10 dB on the IFKP signal strength meter.

The three speeds can be selected at the time that IFKP is chosen from the mode menu. The speed can also be changed using the IFKP modem configuration tab. The quickest way to change speed is to left click on the left-most status indicator on the fldigi status bar (lowest set of controls on the main dialog). A pop-up menu will appear with the speed selections. The quick mode change and the configuration tab changes will occur immediately even during transmission. The selection from the mode menu will cause a complete reset of the IFKP modem.

IFKP Image Mode

fldigi-ifkp has the ability to transmit and receive small digital images selected from those stored on the disk. The image transmission is similar to but not compatible with the MFSKpic modes. IFKP images are always transmitted in a fixed width to height ratio. The W x H format is not limited to a single ratio, nor is it limited to just color, or just gray scale. The image mode is nearly identical to the FSQ image mode with the exception of the RGB transmission sequence. IFKP transmits in Blue-Green-Red order, whereas IFKP transmits in Red-Green-Blue order. The IFKP image bandwidth is less than 400 Hz.

The start of the picture is signalled digitally, and the receiver simply captures data continuously for W lines of H pixels, as requested, resulting in a WxH pixel image, then reception stops.

IFKP Image Mode was designed for NVIS propagation on the lower HF bands. Unlike SSTV, the pictures are sent relatively slowly, in a narrow bandwidth (1/4 of SSTV), and as a result the picture signal to noise ratio is generally better than SSTV for the same transmission power. As a guideline, IFKP signals (before the picture is started) need to be about +10dB SNR for relatively noise-free picture reception.

IFKP pictures also offer significantly better reception than SSTV under NVIS conditions. Because there is no picture sync, there is no risk of the picture tearing due to timing changes, or breaking up during selective fades. All that happens is that the image may noise up momentarily, but will stay correctly aligned. The mode is also significantly less affected by ghosting.

Image Format

The images are approximately in 4:3 or 3:4 aspect ratio. IFKP images can be received by any IFKP capable decoder.

Available image aspect ratios / formats include:

59x74 color160x120 color320x240 color640x480 grey scale640x480 color240x300 color240x300 grey scale120x150 color120x150 grey scaleThese formats correspond roughly to a portrait (3x4) and landscape (4x3) photo. fldigi will scale any computer stored image to the target image. You should edit the images off line to be at least close to the 3x4 or 4x3 ratio. That will prevent the fldigi loader from distorting the loaded image.

The image mode sampling rate is 16,000 samples/sec, the same as the text mode. Each pixel consists of 8 samples, and there are P pixels/line. Therefore the line period is 8 x P/16000 seconds, or the line rate 16000/(8 x P) lines/sec. In colour, each line is sent three times, in Red-Green-Blue (RGB) order.

As an example, if there were 320 pixels per line, and 240 lines, the line rate would be 6.25 lines/sec, and 240 x 3 (for RGB) lines would take 115.2 seconds, or 1.92 minutes for a colour picture containing 76,800 pixels.

There is a single sync signal sent at the beginning of each image. This sync signal is a tone placed 186 Hz below the center frequency, or at 1384 Hertz. The sync signal is 1.5 symbol lengths in duration, or 5.8 seconds.

If the sound cards in the transmitting and receiving computers differ markedly in sample rate, the resulting picture can also be skewed. A small timing difference is enough to cause a noticeable slant in the picture. This is easily fixed after reception using the Slant control. High quality sound cards should be better than 5ppm, and result in barely noticeable slant, but some cheaper computer built-in sound modules may be rather worse. If you plan to send IFKP images you should calibrate your sound card to WWV.

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